This invention relates to the operation of vertical shaft furnaces so as to melt metal, particularly to copper shaft furnaces.
Copper shaft furnaces are used in foundries to melt scrap copper for re-use. Such furnaces are usually in the form of an upright cylinder having towards its lower end a water cooled grating. In use, copper scrap is charged into the furnace to rest on the grating. Below the grating fuel is burned, usually with air, in a substoichiometric quantity in order to produce a hot gas mixture which is not oxidizing towards copper (as any of the copper scrap which becomes oxidized during the melting process is effectively lost). The hot gas mixture passes upwardly through the grating and heats the copper scrap by convection sufficiently to melt the copper. Molten copper percolates downwardly through the scrap charge, and passes through the grating to be collected at the bottom of the furnace. From time to time molten metal is tapped off from the bottom of the furnace into a ladle for use in the foundry. Alternatively, the molten metal may be continuously tapped and collected in a suitable receiver.
A common disadvantage of such furnaces is that the waste gas which issues from the surface of the charge is environmentally harmful. This is particularly the case where the copper scrap is contaminated, with organic waxes or lubricants (such as where the contaminated scrap comprises swarf from a machining operation) for example. Smoke and particulates are usually entrained in the waste gas, which may also contain carbon monoxide, unburnt fuel and hydrogen. It is common to treat the waste gas to render it more environmentally acceptable, however although it is relatively straightforward to remove particulates from the waste gas by filtration, treatment of the waste gas to remove or render harmless the carbon monoxide is difficult and expensive, particularly in the presence of unburnt fuel and/or hydrogen which present a risk of explosion.